Pickling ferrous metal



United States Patent 3,329,619 PICKLING FERROUS METAL Robert M. Hudson, Churchill Borough, and Clair J. Warning, Plum Borough, Pa., assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Aug. 2, 1965, Ser. No. 476,708 4 Claims. (Cl. 252-148) This invention relates to pickling. More particularly, the invention is directed to a pickling composition and to a method of pickling ferrous metal. The term ferrous metal as used herein refers to iron, iron alloys and steel.

To prepare ferrous metal sheet, strip, etc. for subsequent processing, it is frequently desirable to remove [(Samples weight change, in uninhibited acid) (Samples weight change,

(Samples Weight change in uninhibited acid) L: [(Samples hydrogen concentration, uninhibited acid) (Samples hydrogen concentration,

(Samples hydrogen concentration, uninhibited acid) oxide coating, formed during manufacturing, from the surface. The presence of oxide coating, referred to as scale is objectionable when the material is to undergo subsequent processing. Thus, for example, oxide scale must be removed and a clean surface provided if satisfactory results are to be obtained from hot rolled sheet and strip in any operation involving deformation of the product. Similarly, steel prepared for drawing must possess a clean surface and removal of the oxide scale therefrom is essential since the scale tends to shorten drawing-die life as well as destroy the surface smoothness of the finished product. Oxide removal from sheet or strip is also necessary prior to coating operations to permit proper alloying or adherence of the coating to the ferrous metal strip or sheet. Prior to cold reduction, it is necessary that the oxide formed during hot rolling be completely removed to preclude surface irregularities and enable uniform reduction of the work.

The chemical process used to remove oxide from metal surfaces is referred to as pickling." Typical pickling processes involve the use of aqueous acid solutions, usually inorganic acids, into which the metal article is immersed. The acid solution reacts with the oxides to form hydrogen and a salt of the acid. A common problem in this process is overpickling which is a condition resulting when the ferrous metal remains in the pickling solution after the oxide scale is removed from the surface and the pickling solution reacts with the ferrous metal base. An additional difficulty in pickling results from the liberated hydrogen being absorbed by the metal base and causing hydrogen embrittlement. To overcome the aforementioned problems in pickling, it has been customary to add inhibitors to the pickling solution. Although most inhibitors in present use are effective in preventing of overpicklin-g, many do not prevent hydrogen absorption and consequent hydrogen embrittlement. A number of known inhibitors such as some organic compounds containing sulfur have been found to actualy promote hydrogen embrittlement.

The present invention avoids the above-described problems in pickling ferrous metal articles and provides a pickling composition which minimizes both overpickling and hydrogen embrittlement. Thus the pickling inhibitors described herein not only prevent excessive dissolution of the ferrous metal base but effectively limit the amount of hydrogen absorption thereby during pickling. According to the invention, a pickling composition for ferrous metal is provided which comprises a pickling acid such as sulfuric or hydrochloric acid and a small but effective amount of a pyrrole ring compound from the group consisting of pyrrole, pyrrolidine, l-methylpyrrole, 2,5-dimet-hylpyrrole, indole, indoline, and Z-methylindole, 2,5-dimethylindole or 3-pyrroline to inhibit both metal dissolution and hydrogen embrittlement. Ferrous metal articles are pickled with inhibitor)] with inhibitor) 1 The above equations give an indication of the percent inhibition (1) and percent limitation of hydrogen absorption (L). The samples weight change in the uninhibited acid refers to the change in the samples weight due to dissolution by the acid. The samples weight change with 'the inhibitor refers to change in weight of the sample in an acid solution containing the specified pickling inhibitor. All measurements are made after lapse of the same time period and in the data provided herein a standard 4-hour period was used. In the second equation, the samples hydrogen concentration, uninhibited acid refers to the amount of hydrogen that is driven off by warm extraction of the sample, i.e. heating at 160-1 65 C. for 16 hours. The value used in this equation is in cc. of hydrogen/ grams sample. Similarly the samples hydrogen concentration, with inhibitor refers to the amount of hydrogen obtained in the same manner (i.e. by warm extraction) from the sample contacted with pickling acid composition containing a specified pickling inhbitor; also in both these cases for the same unit time, e.g. 4 hours.

If no Weight loss occurs when a speciment is immersed in an inhibited acid then 1:100, which is the maximum inhibitor effectiveness possible for a given acidimmersion and time combination. Similarly, if no hydrogen is absorbed by specimens immersed for a unit time in an inhibited acid, then 11:100. Often an inhibitor will be effective for limiting weigh-t loss during picklng but less eifectve or completely ineffective for preventing hydrogen absorption by the ferrous metal samples. For an inhibitor that increases weight loss I is negative and for one that increases hydrogen absorption L is negative.

The results of a number of tests of various pickling inhibitors including those within the purview of the invention are presented in Tables I and II below. The samples used were of commercially available normalized and temper-rolled low-carbon sheet steel of the following composition: 0.022% carbon, 0.14% manganese, 0.007% phosphorus, 0.017% sulfur, 0.004% silicon, 0.015% copper, 0.002% nickel, 0.010% chromium, 0.002% aluminum, the balance being essentially iron. The samples were 2 x and 0.036" thick and were vapor degreased with trichloroethylene before use. All the samples were pickled at 100 F. for 4 hours by immersion in 2 N hydrochloric acid solution with and without inhibitors (Table I) or 2 N sulfuric acid solution with and without inhibitors (Table II). The samples were weighed prior to pickling and, in the case of the hydrogen absorption test, after extract-ion of the hydrogen absorbed during pickling. The hydrogen concentration in the pickled steel specimens was determined by warm extraction according to the method described by R. M. Hudson et al.,

Journal American Ceramic Society, 41, 23 (1958).

Tables I and II below indicate the relative effectiveness of inhibitors present in difierent concentrations. For some, increasing the quantity of inhibitors does not provide a proportional increase in effectiveness, for others increasing the amount of inhibitors increases their eifectiveness.

Thus for some materials I TABLE I.EFFECTIVENESS OF INHIBITORS FOR PICKLING OF LOW- CARBON STEEL IN 2 N (4-HO UR IMNIE RSION) HYDROCHLORIC ACID SOLUTIONS AT 100 F.

Compound P9999??9999999PP99P999 aNHDNHOlOHO 0 012 Wt Uninhibited- Inhibition Hydrogen Absorption Hydrogen Percent Weight Percent Absorbed, Limitation Loss, mg. Inhibicc. per of Hydroper 0111. tion, I 100 g. gen Absorption, L

0. 3 93 0. 7 84 0.3 91 0. 6 87 0.2 94 0.8 82 1. 1 68 2. 3 48 1. 4 61 2. 3 49 0. 6 84 1. 2 74 1. 6 54 3. 1 1.5 57 2.6 42 1. 2 67 2. 1 53 0. 2 93 O. 4 91 0.4 89 0.8 82 0.1 96 0.2 96 0. 3 91 0. 9 81 0.3 91 0. 6 87 O. 2 93 0. 5 89 0. 1 97 0. 1 98 0. 1 97 0. 3 93 0. 03 99 O. 1 97 0. 9 74 2. 5 0. 3 91 0. 6 87 0. 5 87 1. 1 76 0.7 81 1.6 64 0. 7 1. 3 71 0.2 95 0.4 91

1 Not tested.

TABLE EL-EFFECTIVENESS OF INHIBITORS FOR PICKLING OF LOW- CARBON STEEL IN 2 N SULFURIC A CID SOLUTIONS AT 100 F. (i-HOUR IMME RSION) Inhibition Hydrogen Absorption Compound Concentration Hydrogen Percent Used, percent Weight Percent Absorbed, Limitation Loss, mg. Inhibicc. per of Hydroper cm. tion, I 100 g. gen Absorption, L

0.2 Wt Uninhibited.

1 Not tested.

As can be seen, pickling inhibitors differ greatly in their effectiveness, that is, the I and L values associated with the inhibitor. Moreover, the data illustrate the unpredictability of inhibitor effectiveness since, as shown, some pyrrole ring compounds would not be commercially useful while others are.

If maximum protection against overpickling is desired for a given application, the inhibitors exhibiting I values of 80 or above may be preferred. For many applications, however, the residence time of steel in the pickling bath is relatively short and inhibitors exhibiting lower I values, eg [:40 may be considered useful. In a typical commercial practice, inhibitor concentration on the order of 0.1 volume or weight percent in acid solution would be used. As shown in Tables I and II, many inhibitors can be used at 0.05 volume or weight percent. It is generally desirable to use the least amount of inhibitor which provides the desired results.

It is also known that many materials which are effective in limiting metal dissolution are relatively ineffective in limiting hydrogen absorption, thus there are many instances where high 1 values are coupled with low or poor L values. Inhibitors having minimum L values of from 40 to 50 are acceptable for many applications. For applications in which hydrogen absorbed during pickling would not have an opportunity to escape before subsequent processing, the need for high L value would be greater than for situations where the time lapse following the pickling operation is longer. Thus where steel strips are electrolytically coated immediately after pickling and rinsing, the time lag is slight and pickling inhibitors having relatively high L values should be used.

The quantity of inhibitors used is related to the pickling acid strength. For example, if 0.1 volume or weight percent inhibitor concentration is effective for a 2 N acid solution, then 0.05 volume or weight percent inhibitor concentration should be adequate for 1 N acid solution. In the same Way, 0.2 volume or weight percent inhibitor should be adequate for a 4 N acid solution.

We claim:

1. A pickling composition for ferrous metal consisting essentially of a pickling acid from the group consisting of sulfuric acid and hydrochloric acid and a small but effective amount of a pyrrole ring compound from the group consisting of pyrrole, pyrrolidine, l-methylpyrrole, 2,5-dimethylpyrrole, indole, indoline, and Z-methylindole, 2,5-dimethylindole and 3-pyrroline to inhibit both dissolution and hydrogen embrittlement of said ferrous metal by said pickling acid.

2. A pickling composition according to claim 1 wherein said pickling acid is sulfuric and said pyrrole ring compound is a member from the group consisting of pyrrole, pyrrolidine, l-methylpyrrole, 2,5 -dirnethylpyrrole, indole, indoline and Z-methylindole.

3. A method of pickling a ferrous metal surface comprising contacting said surface with a composition comprising a pickling acid from the group consisting of sulfuric acid and hydrochloric acid and a pyrrole ring compound from the group consisting of pyrrole, pyrrolidine, l-methylpyrrole, 2,5-dimethylpyrrole, indole, indoline, and Z-methylindole, 2.,5-dimethylindole and 3-pyrroline, said pickling composition being effective to remove oxides from said ferrous metal surface with minimum dissolution and hydrogen embrittlement thereof, and washing said ferrous metal to remove said pickling composition.

4. A method according to claim 3 wherein said pickling acid is ulfuric acid and said pyrrole ring compound is a member from the group consisting of pyrrole, pyrrolidine, l-methylpyrrole, 2,5-dimethylpyrrole, indole, indoline and Z-methylindole.

References Cited UNITED STATES PATENTS 1,719,650 7/1929 Chamberlain 252-390 1,746,679 2/1930 Rhodes 252--390 2,043,257 6/ 1936 Missback 252-390 X 3,05 6,746 10 196-2 Brightly 25'2-148 3,214,433 10 1965 Hester 252390 X FOREIGN PATENTS 765,522 1/ 1957 Great Britain.

LEON D. ROSDOL, Primary Examiner.

SAMUEL H. BLECH, JOHN T. FEDIGAN,

Assistant Examiners. 

1. A PICKLING COMPOSITION FOR FERROUS METAL CONSISTING ESSENTIALLY OF A PICKING ACID FROM THE GROUP CONSISTING OF SULFURIC ACID AND HYDROCHLORIC ACID AND A SMALL BUT EFFECTIVE AMOUNT OF PYRROLE RING COMPOUND FROM THE GROUP CONSISTING OF PYRROLE, PYRROLIDINE, 1-METHYLPYRROLE, 2, 5-DIMETHYLPYRROLE, INDOLE, INDOLINE, AND 2-METHYLINDOLE, 2, 5-DIMETHYLINDOLE AND 3-PYRROLINE TO INHIBIT BOTH DISSOLUTION AND HYDROGEN EMBRITTLEMENT OF SAID FERROUS METAL BY SAID PICKLING ACID. 